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The present invention relates to microfabricated structures to interact with electromagnetic waves and, more particularly, to such structures containing pockets and posts for positioning optically anisotropic particles (e.g., rotary balls). Still more particularly, the invention is drawn to addressable, reusable visual displays which may be paper-like, such as xe2x80x9cgyriconxe2x80x9d or twisting-particle displays.
For purpose of illustration, the present application uses structures of gyricon displays to demonstrate the concepts and the benefits of the inventive structure.
A gyricon display, also called a twisting-particle display, rotary ball display, particle display, dipolar particle light valve, etc., offers a technology for making a form of electric paper and other reflective displays. Briefly, a gyricon display is an addressable display made up of a multiplicity of optically anisotropic particles, with each particle being selectively rotatable to present a desired face to an observer. The rotary particle can be of various shapes, such as spherical or cylindrical. For convenience, balls, rather than cylinders, are used in this description for illustrations. Like ordinary paper, electric paper preferably can be written on and erased, can be read in ambient light, and can retain imposed information in the absence of an electric field or other external retaining force. Also like ordinary paper, electric paper preferably can be made in the form of a lightweight, flexible, durable sheet that can be folded or rolled into tubular form about any axis and can be conveniently placed into a shirt or coat pocket and then later retrieved, restraightened, and read substantially without loss of information. Yet unlike ordinary paper, electric paper preferably can be used to display full-motion and changing images as well as still images and text. Thus, it is particularly useful for bistable displays where real-time imagery is not essential, but also adaptable for use in real-time imaging such as a computer display screen or a television.
A gyricon display, also called a twisting-particle display, rotary ball display, particle display, dipolar particle light valve, etc., offers a technology for making a form of electric paper and other reflective displays. Briefly, a gyricon display is an addressable display made up of a multiplicity of optically anisotropic particles, with each particle being selectively rotatable to present a desired face to an observer. The rotary particle can be of various shapes, such as spherical or cylindrical. For convenience, balls, rather than cylinders, are used in this description for illustrations.
In the prior art, the black-and-white balls (particles) are embedded in a sheet of optically transparent material, such as an elastomer sheet. The elastomer sheet is then cured. After curing, the elastomer sheet is placed in a plasticizer liquid, such as a dielectric fluid. The dielectric plasticizer swells the elastomer sheet containing the particles creating cavities larger than the particles around the particles. The cavities are also filled with the absorbed dielectric fluid. The fluid-filled cavities accommodate the particles, one particle per cavity, so as to prevent the particles from migrating within the sheet.
Besides being optically anisotropic, the particles are electrically dipolar in the presence of the fluid. This may be accomplished by simply including in one or both hemispheres materials that impart an electrical anisotropy, or by coating one or both sides of hemispheres with materials that impart electrical anisotropy. The above charge activation agents may impart an electrical anisotropy and an optical anisotropy at the same time. For example, when each hemisphere of a gyricon particle is coated with a material of a distinct electrical characteristic (e.g., Zeta potential with respect to a dielectric fluid) corresponding to a distinct optical characteristic the particles will have an electrical anisotropy in addition to their optical anisotropy when dispersed in a dielectric liquid. It is so because when dispersed in a dielectric liquid the particles acquire an electric charge related to the Zeta potential of their surface coating.
An optically anisotropic particle can be selectively rotated within its respective fluid-filled cavity, for example by application of an electric field, so as to present either its black or white hemisphere to an observer viewing the surface of the sheet. Under the action of an addressing electric field, such as provided by a conventional matrix addressing scheme, selected ones of the optically and electrically anisotropic particles are made to rotate or otherwise shift their orientation within their cavities to provide a display by the selective absorption and reflection of ambient light. In the case of balls with black and white hemispheres, for example, an image is formed by the pattern collectively created by each individual black and white hemisphere. Thus, by the application of an electric field addressable in two dimensions (as by a matrix addressing scheme), the black and white sides of the particles can be caused to appear as the image elements (e.g., pixels or subpixels) of a displayed image. Since the particles need only rotate, not translate, to provide an image, much faster imaging response is achieved than with the display of U.S. Pat. No. 3,612,758.
Normally, the particles are kept from rotating freely by a certain degree of frictional adhesion to the cavities. When the electric field is applied to the sheet, the adhesion of each particle to the cavity is overcome and the particles are rotated to point either their black or white hemispheres towards the transparent surface. Even after the electric field is removed, the structure (i.e., particles arranged in a specific pattern of orientations) will stay in position and thus create a bistable display until the particles are dislodged by another electric field. These bistable displays are particularly useful for remotely addressable displays that require little power to switch and no power to maintain display image for a long period of time (e.g., months).
Gyricon display technology is described further in U.S. Pat. No. 4,126,854 (Sheridon, xe2x80x9cTwisting Ball Panel Displayxe2x80x9d) and U.S. Pat. No. 5,389,945 (Sheridon, xe2x80x9cWriting System Including Paper-Like Digitally Addressed Media and Addressing Device Thereforxe2x80x9d). Further advances in black and white gyricon displays have been described in U.S. Pat. No. 6,055,091 (Sheridon, xe2x80x9cTwisting-Cylinder Displayxe2x80x9d). The above-identified patents are all hereby incorporated by reference. The Sheridon patent disclosed a gyricon display which uses substantially cylindrical bichromal particles rotatably disposed in a substrate. The twisting cylinder display has certain advantages over the rotating ball gyricon because the elements can achieve a much higher packing density. The higher packing density leads to improvements in the brightness of the twisting cylinder display as compared to the rotating ball gyricon.
Gyricon displays are not limited to black and white images, as gyricon and other display mediums are known in the art to have incorporated color. Gyricons incorporating color have been described in U.S. Pat. No. 5,760,761 titled xe2x80x9cHighlight Color Twisting Ball Displayxe2x80x9d, U.S. Pat. No. 5,751,268 titled xe2x80x9cPseudo-Four Color Twisting Ball Displayxe2x80x9d, U.S. patent application Ser. No. 08/572,820 titled xe2x80x9cAdditive Color Transmissive Twisting Ball Displayxe2x80x9d, U.S. patent application Ser. No. 08/572,780 titled xe2x80x9cSubtractive Color Twisting Ball Displayxe2x80x9d, U.S. Pat. No. 5,737,115 titled xe2x80x9cAdditive Color Tristate Light Valve Twisting Ball Displayxe2x80x9d, U.S. Pat. No. 6,128,124 titled xe2x80x9cAdditive Color Electric Paper Without Registration or Alignment of Individual Elementsxe2x80x9d and European Patent No. EP0902410 titled xe2x80x9cMethods for Making Spinnable Ball, Display Medium and Display Devicexe2x80x9d. The above-identified patents are all hereby incorporated by reference.
The above prior art all involve a process which is to randomly pack the bichromal particles in an elastomeric matrix, cure the elastomer, and subsequently swell the elastomer in the dielectric oil. The process is laborious and time-consuming, consisting of mixing of the particles into the elastomer, coating the slurry into a sheet format, curing, and subsequently swelling the sheet with the dielectric oil.
Furthermore, the display device of such an arrangement poses problems in connection with the selection of a usable dielectric liquid, stability upon changes in temperature, non-uniformity of dimensions of the cavities, and the like. The material considerations in the prior art are many, the primary issues being tuning the swelling of the elastomer by the dielectric oil without harming the dielectric oil compatibility with all the other elements of the display package.
Furthermore, the above approach resulted in less than satisfactory contrast of the display, associated with the relatively low reflectance of a gyricon display. It is commonly believed that the best way to improve the reflectance of a gyricon display is to make the display from a close packed arrangement of bichromal particles. The closer packed the arrangement of particles, the better the reflectance and the brighter the appearance of the display. To achieve a close packed arrangement, the cavities in which the particles rotate should be close to each other and each cavity should have little unfilled space when filled with a particle, ideally no more empty space than what is necessary to keep the particle therein rotatable. The prior art approaches, however, had difficulties to achieve a high density of particles, mainly due to the lack of controlling on the formation of individual cavities. The result is typically that cavities are either too large, or distributed too loosely in the elastomer with large distances and thick walls between the individual cavities, making it difficult to control the arrangement and packing density of the display particle members to a sufficiently high value to achieve a display of high quality, high resolution, and high contrast.
As a related problem, in a typical conventional gyricon display, bichromal particles are dispersed throughout the thickness of the substrate sheet, which is always thicker than two particle diameters and is usually many diameters thick. Generally, less than 20 percent of the upper surface area of the sheet is covered by the bichromal particles in the layer closest to the surface. Therefore, a display according to the above prior art has multiple layers of particles instead of a single layer, making the display thick and bulky, an undesirable feature especially for an electronic paper. In the prior art designs, the multiple layer configuration is on one hand necessary in order to increase the reflectance (the reflectance of multiple layers of loosely packed particles accumulatively approaches that of a closely packed single layer) and on the other hand difficult to avoid due to the characteristics of the prior art process of making a display.
To achieve higher packing density, the above method was modified in U.S. Pat. No. 4,438,160 to Ishikawa et al, which patent is incorporated by reference. In the Ishikawa patent, instead of using the swelling method to create cavities larger than the particles, the particles are coated with a layer of wax before being placed in the elastomer. The wax is later melted away, resulting in cavities that are larger than the particles. Presumably, because it is easier to control the thickness of the wax layer coated on the particles than to control the degree of swelling the elastomer, it is also easier to achieve higher density of particles by using the Ishikawa method. The actual improvement, however, is not significant enough to solve the problem. See U.S. Pat. No. 5,825,529 to Crowley, which patent is incorporated by reference.
To achieve still higher packing density, a gyricon display can be constructed without elastomer and without cavities. U.S. Pat. No. 5,825,529 to Crowley, for example, uses no elastomer substrate. In the display in the Crowley patent, the bichromal particles are placed directly in the dielectric fluid. The particles and the dielectric fluid are then sandwiched between two retaining members (e.g., between the addressing electrodes). There is no elastomer substrate. Electrodes serve both to address particles and to retain particles and fluid in place. Particles and fluid can be sealed in the display by seals at either end of the display. In addition, the spacing between electrodes is set to be as close to the diameter of particles as is possible consistent with proper particle rotation, resulting a monolayer display. The Crowley patent achieved a display with a closely packed monolayer having a light reflectance that surpasses that of the multi-layer displays in the prior art.
The display in the Crowley patent, however, has two potential problems. First, the display package is environmentally and mechanically sealed only around the perimeter of the display. This results in the package being susceptible to cracking as may result from wear and tear; in this instance, a single crack would be adequate to enable all of the dielectric oil to drain or evaporate away, thereby disabling the function of the display. Additionally, the package is susceptible to buckling, and the elastomer-particle film can sag or slide out of position because of gravity (especially when held vertically for long periods of time, such as for display signs). This is because the mechanical support for the package is primarily the thin polymer films on the front and back sides, and because the reinforcement of these films occurs only where they are bonded together along the periphery.
The invention described herein circumvents all of the above-identified concerns existing in the prior art. As compared to random-packed particles in an elastomer matrix, the invention uses no elastomer matrix. The invention uses pre-formed pockets and/or posts to organize particles in a monolayer close packed array. The post and pocket film structure eliminates need for elastomer, thus eliminates the slowest process steps (curing elastomer and swelling elastomer with dielectric oil). As compared to the Crowley patent, the particles in the present invention are supported by posts and/or pockets and further sealed separately in individual pockets. Furthermore, where an electrode plate or a cover film is used, such as in an addressable gyricon visual display, the electrode plate or a cover film may be supported by the ends of posts and/or the sidewalls of pockets to strengthen the structure.